ASP.NET Core 的 ActionFilterAttribute 中的异步 OnActionExecuting

Question

ASP.NET Core's ActionFilterAttribute has these:

public virtual void OnActionExecuting(ActionExecutingContext context);
public virtual void OnActionExecuted(ActionExecutedContext context);
public virtual Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next);

I need an async version of OnActionExecuting , which doesn't exist.

However I have a feeling that I can use OnActionExecutionAsync instead, as it also has an argument of ActionExecutingContext .

Am I correct that despite the name, they trigger at the same point in the process?

Also, what do I need to do with the next argument? Once I'm done with my stuff, do I simply need to call await next() ?

Is that it? I'm unsure as I can't find docs for this.

Answer 1

Asynchronous filters work a bit differently: first execute code that must be executed before the action, call next() for the actual logic, finally add code to be executed after the action.

public async Task OnActionExecutionAsync(ActionExecutingContext context, 
                                         ActionExecutionDelegate next)
{

    // logic before action goes here

    await next(); // the actual action

    // logic after the action goes here
}

The documentation is here: https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers/filters#implementation

Answer 2

Asynchronous filters always take precedence over the synchronous filter implementations.

According to the Docs :

• It's advised to implement either the synchronous or the async version of a filter interface, not both. The runtime checks first to see if the filter implements the async interface, and if so, it calls that. If not, it calls the synchronous interface's method(s). If both asynchronous and synchronous interfaces are implemented in one class, only the async method is called.

However, you can manage to have both. For instance:

public class TimestampFilter : IActionFilter, IAsyncActionFilter 
{    
    public void OnActionExecuting(ActionExecutingContext context)    
    {         
        context.ActionDescriptor.RouteValues["timestamp"] = DateTime.Now.ToString();    
    }

    public void OnActionExecuted(ActionExecutedContext context)    
    {         
        var ts = DateTime.Parse(context.ActionDescriptor. RouteValues["timestamp"]).AddHours(1).ToString();        
        context.HttpContext.Response.Headers["X-EXPIRY-TIMESTAMP"] = ts;    
    }

     public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)    
    {        
        this.OnActionExecuting(context);        
        var resultContext = await next();
        this.OnActionExecuted(resultContext);    
    }
 }

Answer 3

An ever better pattern:

public override async Task OnActionExecutionAsync(
    ActionExecutingContext context, 
    ActionExecutionDelegate next)
{
    try
    {
          //do your async things here                 
    }
    finally
    {
        await base.OnActionExecutionAsync(context, next); <--- notice this!
    }
}